If Trump moves forward with the tariffs, Malmstr-m said the EU would challenge them at the World Trade Organization (WTO).

The US has sought to justify its plan on national security grounds. Malmstr-m said the EU has serious doubts about that rationale, describing the metal tariffs as "an economic safeguard measure in disguise."

Malmstr-m said that any response from Europe would be "proportionate" and comply with WTO rules. She said new tariffs could be implemented in a matter of months.

With tensions running high between the US and EU, analysts have warned that the spat could devolve into a tit-for-tat trade war that would cause prices to rise and damage economic growth.

Trump threatened over the weekend to respond to any new EU trade barriers with a tax on vehicles made by European carmakers.

"The truth is trade wars are bad and easy to lose," said Tusk, who added that the EU's goal was to "keep world trade alive."

The 28 nations in the EU produce 10% of the world's steel. EU trade officials are worried that jobs would be at risk if steel that would have otherwise been sold in the US is diverted to Europe, pushing down prices.

The US is the world's top steel importer. The value of steel shipped into the US was just over $29 billion in 2017.

China, which produces roughly half of the world's steel, has in the past been accused of dumping the metal on other markets, pushing down prices, forcing rivals out of business and killing thousands of jobs.